The present invention relates to a dry-etching method making use of a glow discharge plasma, and more particularly to a method for selectively etching a material for a semiconductor device such as silicon, molybdenum, tungsten, chromium, TiW, silicon dioxide, phosphosilicate glass or silicon nitride by the use of a plasma etcher having a parallel plate electrode structure.
Recently, the sizes of the electronic parts of a semiconductor integrated circuit or the like are becoming finer and finer, and the technique of the so-called "dry-etching method" making use of gas plasmas has accordingly made rapid progress as the technique which can work a selected material highly accurately and finely, in place of the conventional wet-etching method using a liquid. Especially, in the plasma etcher having a parallel plate electrode structure, the selective etching of Al, an Al alloy, silicon dioxide and so on can be made, and practical investigations of this process are being advanced.
Upon the fabrication of the semiconductor integrated circuit or the like, incidentally, a silicon compound such as silicon oxide, phosphosilicate glass or silicon nitride is widely used as an electrically insulating film for separation of elements and between layers, a mask for etching, diffusion, oxidization or ion implantation, or a protecting film. In accordance with the prior art, as a gas for dry-etching these silicon compounds, or silicon polycrystal or molybdenum to be used as the gate material for a MOS transistor, there are used not only a fluorocarbon having a structure of C.sub.n F.sub.2n+2 represented by CF.sub.4, and the fluorocarbon gas mixed with O.sub.2 or an inert gas but also a fluorocarbon-containing gas such as CCl.sub.2 F.sub.2, CHF.sub.3 or CBrF.sub.3 or a fluorine-containing compond such as SF.sub.6, BF.sub.3, SiF.sub.4 or NF.sub.3. However, all of those gases are expensive, and the use of such expensive gases raises an obstacle against the reduction in the cost for the dry-etching method.
For the purpose of practically using the dry-etching in the semiconductor fabricating process, on the other hand, it may be an important factor to etch a material at a higher rate possible than with a photoresist material providing an etching mask therefor on a surface for the material to be etched, i.e., to have a high level in the so-called "selectivity". Generally speaking, if a silicon compound such as a silicon dioxide film is dry-etched by the use of the aforementioned fluorocarbon-containing gas, the etching rate of the silicon dioxide is either lower than that of the photoresist or the silicon, or is not a practically sufficient rate even in case the rate is high. As a method for enhancing the selectivity, therefore, there has been found either a method, in which the aforementioned fluorocarbon gas is mixed with a gas containing hydrogen such as H.sub.2, H.sub.2 O, NH.sub.3 or C.sub.2 H.sub.4, or a method in which a fluorocarbon gas is introduced and used as a gas for dry-etching and in which a high molecular material such as polyester or an ethylene tetrafluoride resin is arranged on the inner wall of a reaction chamber or on a target electrode (e.g., an r.f. electrode). In accordance with that method, fluorine radicals, which are dissociated under discharge from the fluorocarbon gas and which act as a highly reactive chemical etchant, are made to react with the hydrogen atoms in the mixed gas or in the target material and further with the (CF.sub.n)-containing gas, which is generated from the target material, so that they are caused to restrain their reactions with the silicon or the photoresist thereby to selectively etch the silicon compound such as the silicon dioxide with a fraction of CF.sub.2 or CF.sub.3.
However, the method thus far described raises problems that the plasma polymerization under the plasma discharge becomes prominent to blot the surface of the sample or the inside of the reactor and that it becomes liable to reduce the etching rate of the silicon compound or to deteriorate the photoresist. Especially, the deterioration in the photoresist is an important problem and is followed by deformation so that the patterning accuracy is degraded to invite such practically serious obstacles that it becomes impossible to etch fine patterns less than 5 .mu.m and difficult to remove the photoresist after the etching process. In the plasma etcher of parallel plates, generally speaking, the deterioration of the photoresist is the more liable to take place for the lower gas pressure and for the higher power density impressed, and the temperature rise on the surface of the sample as a result of the ion impact is considered to be one of the causes therefor. Therefore, if the gas pressure is increased and if the power density is decreased, it is possible to prevent that deterioration. In this particular case, however, the etching rate of the silicon compound is lowered so that the selectivity is remarkably reduced. On the other hand, in case the fluorocarbon-containing gas is used without mixing the gas containing the hydrogen atoms, the selectivity is improved the more for the higher molecular gas such as C.sub.3 F.sub.8 or C.sub.4 F.sub.8 than C.sub.2 F.sub.6 or C.sub.2 F.sub.6, and the far more than CF.sub. 4 or for a gas containing hydrogen atoms such as CHF.sub.3. In that particular case, on the contrary, the deterioration of the photoresist is liable to take place, and the etching rate of the silicon compond is reduced. If the selectivity is to be improved at a practical etching rate in this manner, an adverse result is obtained in that the deterioration of the photoresist is liable to take place.
In order to prevent the photoresist from being deteriorated, incidentally, there has been proposed a method in which the thermal contact between a water-cooled target electrode and a sample is improved. This method is effective in more or less reducing that deterioration, but cannot substantially solve the problem. There are still left practical problems in workability, reproducibility and contamination in the etcher.
It is, therefore, the present technical state that there is no etching method capable of attaining both practially sufficient selectivity (the ratio in the etching rate of the material to be etched to another material is higher then ten times) and etching rate by the use of the fluorine compound or its gas mixed with another gas without being accompanied by the deterioration of the photoresist.
On the other hand, the special fluorocarbon gas having a relatively excellent selectivity such as C.sub.2 F.sub.6, C.sub.3 F.sub.8, C.sub.4 F.sub.8 or CHF.sub.3 is difficult to obtain and costs ten times or more as high as the later-described He or Ar gas which is to be used in the present invention, thus raising a serious problem when it is used in the mass-production process of a semiconductor.
The following references are cited to show the state of the art:
(i) Japanese Patent Laid-Open Publication No. 49-2482;
(ii) Japanese Patent Laid-Open Publication No. 52-114444;
(iii) Japanese Patent Laid-Open Publication No. 52-131469.